The invention relates to a safety device comprising a stop for a drilling instrument of use inter alia in dental surgery. The invention also relates to a device for precalibrating and storing the depth of drilling and directly related to an advantageous process of using the safety device.
One preferred application of these devices is to dental implantology.
As is known, during some operations, inter alia the treatment of root canals, some dental lesions, hard dental or maxillary tissue (e.g. when fitting a dental implant) the depth of penetration, drilling or boring by a specific instrument (endodontic instrument, dental bur or drill, implantology drill bit, etc) must be to a clearly defined depth determined by the practitioner as required by the patient.
The defined depth must be actually obtained but not exceeded. This condition is particularly important in dental implantology, where it is essential to ensure the long-term reliability of the infrastructure for inserting into the jawbone in order to hold a fixed dental prosthesis, but without any risk of injury to neighbouring structures.
For these reasons, manufacturers propose various devices for conforming to these constraints.
For example, in a very simple but very frequently-used device in dental implantology, the instrument has markings in the form of grooves, coloured or otherwise, at different distances P1, P2, . . . Pn on the active part, i.e. the cutting part of the drill bit, so that the drilling operation can be stopped at the right moment, when the practitioner sees that the previously-determined depth has been reached (visual marks, reference 14 in the accompanying FIG. 1).
In other more sophisticated devices (which will be briefly described hereinafter) the instruments are provided with stops, although it should be noted that not one of these prior-art devices is suitable for dental implantology nor intended for use in this special sector.
EP-0 515 274 describes an instrument (a helical drill bit) used in dental surgery and having a cutting part 7 formed with transverse grooves which can serve as an optical reference part indicating the depth of drilling. The practitioner can place a stop ring (8, 9) on the appropriate groove, i.e. the groove corresponding to the depth of the hole which he intends to make, so that he will be better able to check the depth of drilling by positioning the stop, which will thus remain fixed during the required operation.
The teaching in FR-2 613 212 is similar except that the non-operative part 1b has a number of peripheral transverse notches for positioning a clip constituting a fixed depth stop for determining and checking the length of that part of the instrument which is to penetrate into the tooth.
In a variant disclosed in U.S. Pat. No. 4,526,542, the stop is screwed to the drill shank until locked against the rear surface of the cutting part, the screwthread or internal screwthread being oriented oppositely to the rotation of the drill, so that the stop serves as a lock-nut. When the stop comes in contact with a neighbouring surface it will tend to be locked more tightly on to the shoulder presented by the working part at the top. As we shall see hereinafter, this is contrary to the aim of the inventor of the present invention.
The International Application WO 98/03125 discloses a drill bit having a working part formed with calibrated recesses spaced apart (relative to the drill axis) by a millimeter in each case. An annular stop can be moved along the said working part and can be immobilised at each recess, when a pin disposed in the stop can engage in a recess under the action of a spring.
These devices can display the depth of drilling or boring intended by the practitioner, i.e. the stop on the instrument can be positioned in dependence on the said depth, and also serves as a safety element since the aim, once the instrument has been adjusted, is to ensure that the said depth is not exceeded during the operation.
All these known stop devices, however, have at least three kinds of disadvantages as regards the specialist in dental implantology who wishes to drill for the purpose of fitting implants. Firstly, insofar as a stop is simply fixed at a predetermined working position, the prior art devices to not always make optimum use of the depth of the site of operation, apart from the fact that the accuracy of depth display is unsatisfactory. Secondly, in cases where the working conditions are not optimum, these devices may produce an oval hole and thus endanger the primary stability of the implant, in which case a satisfactory result cannot be guaranteed, particularly as regards the stability of the implant (see hereinafter). Finally the prior art devices are uneconomic.
Although these three groups of disadvantages can easily be understood by examining the prior publications, it is worth explaining them further:
When the instrument, which by definition is subjected to pressure during operation, reaches the defined drilling depth, the stop may slip on meeting a surface inclined with respect to the axis of the instrument or an irregular surface, concave or convex at places (which is very frequently the case in implantology, where dental implants have to be fitted on to an apex of a bone crest only slightly larger in diameter than the implant). The probability of slipping is far from negligible, and is greater in cases where the bone for drilling is very spongy and loose or in the very frequent cases where the uppermost part of the apex of the bone crest (always on the same side as the tongue) is of hard cortical bone whereas the lower part (on the buccal side) is softer (only slightly corticalised). The result of such an eventuality will inevitably be a side slip, i.e. a lateral drift of the drill axis, and a consequently oval hole.
This serious disadvantage of the known devices is one reason for lack of primary stability of the implant.
Techniques and knowledge in implantology are in process of development. One development is towards use of non-traumatic techniques, when preparing holes in bones for receiving xe2x80x9cscrew implants in root formxe2x80x9d. Another development is towards adapting drilling techniques to the four bone densities (type D1 comparable to oak wood down to type D4, corresponding to expanded polystyrene) normally found in dental implantology. Consequently the dentist, or more precisely the implantologist, will need a very large number of preferably increasing drill diameters (usually from 1.5 to 5 mm). This is to prevent injury through heating of the dense bone by too rapid drilling of the hole. Another purpose is to adapt the preparatory work to the different diameters of the implants at present available. A final aim, in the case of spongy or loose bone, is to produce holes sufficiently tight for inserting an implant with good primary stability without risk of fracturing the bone trabiculation when inserting an implant, through insufficient drilling of the hole in the bone.
These situations are not taken into account by any of the known safety devices for limiting the depth of drilling.
Finally no citation discloses means for precalibrating the planned depth of drilling and storing the said depth.
The object of the invention is to reduce the said disadvantages and take account of the considerations stated.
The object is achieved by the device and method defined in the claims.
The claimed stop, in addition to being mobile, can be described as xe2x80x9cactivexe2x80x9d in contrast to the xe2x80x9cpassivexe2x80x9d character of the stops known in the prior art, in that the co-operation as described between the stop and the drilling tool or instrument inevitably and automatically results in withdrawal of the instrument from the hole just made by the practitioner, as soon as the desired depth is reached. The moving stop positioned according to the invention effectively prevents the preset drilling depth from being exceeded, together with any other risks such as ovalisation or lesions.
To sum up, as soon as the stop meets an obstacle and is thus prevented from rotating, the stop moves exclusively in the direction for shortening the length of the exposed working part of the instrument, thus avoiding any accident.
Likewise if, when the instrument is set in rotation, the inertia of the stop causes it to rotate on the co-operating means (the screw and nut system in one example) connecting these two elements (i.e. the stop and the instrument, more particularly a drill or drill bit), the resulting rotation communicated to the stop will involve axial displacement thereof in a direction such that the displacement will shorten the portion of the working part of the tool initially projecting from the stop. This is a safety factor, particularly in dental implantology.
The automatic withdrawal of the drill bit at the end of travel eliminates any risk of drift by the tool, i.e. any risk of ovalisation of the hole due to side-slipping of the stop. This feature, as we have seen, is of fundamental importance in dental implantology, particularly in the case of slightly corticalised bone.
Retraction of the drill bit also prevents any excess drilling during the preparation of a hole in very spongy bone (usually in the posterior part of the jaw bones, particularly the upper jawbone). It is essential to avoid any excessive drilling of the tender bone. The lack of accuracy resulting from excessive drilling will destroy the primary stability of the implant, such stability being essential for incorporation in the bone (i.e. the creation of a stable biological link between the bone and the implant so that it can bear loads efficiently and over a longer period without clinical symptoms. This is a fundamental principle and aim of modern implantology).
The adjustment of the depth of drilling by the prior-art stop devices, using bearings or notches, is rudimentary and insufficiently accurate. By contrast, in the device according to the invention, the stop is connected to the non-operative part (shank) of the instrument, preferably via a micro-screwthread, permitting easy, fine and accurate micrometric adjustment of the depth of drilling.
The advantage of micrometric adjustment, by precalibrating the depth of drilling, using an X-ray or tomographic film to scale 1:1, is that optimum use is made of the depth of bone available, without risk of injury to neighbouring structures. The chosen depth of drilling can be adjusted easily during the operation, by very accurate micrometric movement of the stop, leaving the drill in the bone hole under X-ray monitoring. In the case of digital radiography, the depth can be checked and corrected if required (i.e. the stop can be re-positioned) immediately, with the additional advantage of better use of time. Finally the micro-screwthread is a means of accurately correcting and compensating the error due to prestress of the edge of the stop by the nearest part of the bone surface (the position of a stop is pre-adjusted relative to the drill axisxe2x80x94an impossible operation in the case of a stop with bearings or notches).
This method also has an economic advantage, since the standard diameter of shanks (for different diameters of the working part) of drills used in implantology is always 2.2 millimeters, so that a standard stop can be used for a whole range of drills. In other applications where necessary, the standard stop can be supplemented by stops adapted to other shank diameters. In all cases, the device according to the invention enables the number of stops to be narrowly limited, perhaps to a single standard type covering practically all sizes of drill (a feature which makes these devices all the more advantageous in sectors such as implantology where wear is particularly important).